1. The Field of the Invention
The invention relates to the detection of distance to an object surface. More particularly, the invention relates to a triangulation displacement sensor for the detection of distance to an object surface.
2. The Relevant Technology
The technique of triangulating distance to an object using lasers and multi-element detector linear arrays has been practiced and published since at least 1978. See, for example, I. McFarlane, Development of the Heightscan Thickness Gauge, SPIE Vol. 145, Sira, London (Mar. 14, 1978); J. Waters, Gaging by Remote Image Tracking, SPIE Vol. 153, Advances in Optical Metrology, (Aug. 28-29, 1978); and Wagner, U.S. Pat. No. 4,040,738. Distance measurement is accomplished when an optical beam is projected out from a source and strikes the object surface. The beam is then viewed by a camera that is displaced from the axis of projection of the beam by some baseline distance. The camera is angled so that the laser beam crosses the field of view of the camera. When the beam strikes a surface at a point within the field of view of the camera, light reflected from that point is typically imaged by a lens onto the camera""s detector.
The detector may be either a continuous device such as a position sensing detector (PSD), which generates an electrical signal proportional to the position of the spot image on the PSD, or a linear charge coupled device (CCD) array, which consists of a single line of photodiode detector elements, each of which generates an electrical signal in proportion to the amount of light falling on it.
The signal from the camera is typically processed by a microprocessor or other electronic logic which determines the location of peak light intensity on the camera, and a calibration table and/or equation is used to translate this location among the camera""s pixels to a distance from the sensor. The data is then output in a form that can be read and used by computers or displayed visually.
In the case of a linear array detector, if more than one pixel is illuminated by the image of the spot, the center of gravity, or centroid, of the spot image may be calculated to a position with resolution corresponding to a fraction of the size of a single camera pixel. Thus if the image of the spot is larger than a single pixel, the position of the object may be determined to a resolution better than that corresponding to the pixel spacing of the camera.
At the same time, it is desirable to use a small, well defined spot of light on the target so that small surface features may be resolved (see, for example, McFarlane, Waters, and Wagner, supra.). In many configurations, the small spot at the target combined with the high quality imaging lenses results in a small image spot on the detector, and the pixels of the camera must be correspondingly limited in size to facilitate centroid finding with multiple pixels illuminated. Enlarging the spot image by placing the detector array at a point other than at the image focus creates a larger illuminated area on the array, but magnifies irregular features in the image, as described below.
It is common to use a laser (see, for example, Waters, supra. and Pryor et al, U.S. Pat. No. 4,373,804) or laser diode (T. Clarke, K. Grattan, N. Lindsey, Laser-based Triangulation Techniques in Optical Inspection of Industrial Structures, SPIE Vol. 1332 Optical Testing and Metrology III: Recent Advances in Industrial Optical Inspection (1990) and Wagner, supra) light source in triangulation sensors because light from lasers may be focused to a small spot and may provide high intensity. The use of a laser introduces the difficulty that the image of the spot contains high frequency laser speckle which must be filtered out (see, for example, Pryor, supra.). Additionally, some types of laser diodes do not generate smooth intensity profiles, the light intensity being irregular and varying across the output beam. This is then evident in the image of the spot on the detector array. Finally, fine-pitch variations in reflectance from the target surface may cause the image of the spot to have irregularities.
All of the above factors create a spot image that is irregular, rather than having an intensity rising to a smooth peak and then falling off in a symmetric fashion. These irregularities can lead to inaccuracies in determining the location of the centroid of the spot, with resultant measurement error.
It would be desirable to provide an optical sensor that produces a spot image having an intensity rising to a smooth peak and then falling off in a symmetric fashion. It would also be desirable to provide a means of controlling the image spot size at the detector independent of the geometry and components of the rest of the apparatus.
The invention provides an improved optical sensor for detecting the distance to an object surface based on triangulation. The sensor projects a beam from a laser diode through a focusing lens onto the object surface. The laser may be a VCSEL (Vertical Cavity Surface Emitting Laser) diode, which affords advantages in beam quality, intensity noise, and power consumption relative to other laser diodes.
Light scattered or reflected from the target surface is collected by a collection lens and passed through a beam shaping or homogenizing element which removes undesired intensity variations that occur in the spot at the target surface. These variations may be caused by laser speckle, structure in the output beam intensity distribution, and/or nonuniform reflectance of different regions of the object surface illuminated by the spot.
The homogenized beam then strikes a CMOS linear array detector, which converts the light intensity on its detection elements to electrical levels which are read sequentially. The detector elements are larger than an image of the spot on the array would be, but an image is not formed on the array. Instead, the beam shaping element enlarges the size of light slightly, and mixes, or homogenizes, the light. This destroys the image and the intensity variations originating at the spot on the surface that would be contained in the image.
The enlargement of the area of illumination together with the homogenization allow the position of the beam on the array to be determined to a resolution of about {fraction (1/10)}th of the width of a detector element. This position is then converted to an absolute distance output which may be read by digital or analog means.
These and other features, and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.